[Fix] fix attention clamp max params

[yingfhu]

Motivation

The original clamp operation is hard to read.

Modification

Use np.log instead for better compatibilities for all pytorch version

Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with those projects, like MMDet or MMSeg.
• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 86.14% // Head: 86.14% // No change to project coverage 👍

Coverage data is based on head (2ba3e89) compared to base (0b4a67d).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1034   +/-   ##
=======================================
  Coverage   86.14%   86.14%           
=======================================
  Files         140      140           
  Lines        9678     9678           
  Branches     1675     1675           
=======================================
  Hits         8337     8337           
  Misses       1090     1090           
  Partials      251      251           

Flag	Coverage Δ
unittests	86.07% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcls/models/utils/attention.py	96.91% <ø> (ø)

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